WASHINGTON — A new kind of genetic sleuthing suggests hospital outbreaks of drug-resistant staph bacteria don't always spread from one patient to another, but that numerous people — patients, visitors or staff — bring in the deadly germ.
The finding came as British scientists used in-depth gene scanning to track how a dangerous strain of this bacteria, called MRSA, has spread around the world.
The result, published Thursday in the journal Science: practical evidence about why some hospital defenses fall short and a possible new tool to help figure out better ones.
"To the lay public, MRSA just means one big, bad, ugly organism," said Dr. Buddy Creech, a Vanderbilt University infectious disease specialist who wasn't involved in the research but called it an important step because it uncovers multiple subtypes that standard testing can't.
"It proves to us that not all MRSA are created equally," he said.
MRSA stands for methicillin-resistant Staphylococcus aureus, a form of the incredibly common staph family of germs. About one in every three people carries staph aureus in their noses with no symptoms but still can infect others; about 1 million people in the United States carry the MRSA type.
The germs usually cause skin infections but can be deadly if they penetrate the bloodstream or organs, and MRSA is blamed for about 18,000 U.S. deaths a year.
Older tests hunt for a handful of bacterial mutations to classify MRSA into different strains.
Scientists at Britain's Wellcome Trust Sanger Institute went a big step further: They created genetic blueprints of 63 samples of MRSA collected around the world from 1982 to 2003, and tracked even minor DNA variation to trace how one common strain, called ST239, spread from continent to continent and within one hospital in Thailand.
The resulting family tree suggests this MRSA strain originated in Europe in the 1960s, just as antibiotics were being widely used. It spread to South America and became dominant in parts of Asia.
Perhaps most eye-opening: Of 20 ST239 patients during seven months in that Thai hospital, five shared the exact infection. The germs' genetic diversity showed the rest were coincidentally brought into the hospital.
Hospital-acquired MRSA has long been thought to be more dangerous than MRSA caught in ordinary community settings, perhaps because the hospitalized are more vulnerable.
This study shows "there's a blurring in distinction" between the two, said study co-author Dr. Sharon Peacock of the University of Cambridge.
Hospital efforts such as hand-washing and room-cleaning help only so much if MRSA spreaders keep walking in the front door, she said.